I often correspond in French and recently discovered (by accident!) a quick way to add symbols such as accent (acute and grave), circumflex, tilde and umlaut to letters while typing, without having to open a third party application or memorize a new keyboard shortcut.
To see the array of accents and symbols available for a specific key, simply press and hold down the key that you want to mark with an accent. A small pop-up menu of accented versions of the letter will pop up. To select the accented letter you want, click it with your mouse or select it with your arrow keys on your keyboard and then press Return.
The letters that have accents and symbols available are:
Thus far, this worked in every note-taking or word processing application I have tried.